All motile prokaryotes use essentially the same mechanism to monitor the chemistry of their surroundings and navigate toward favorable environmental conditions. The E. coli chemotaxis system is the best-characterized example. Sensory-motor regulation is mediated by a densely packed receptor array that is imbedded in a patch of membrane at one pole of the cell. The structure is a fibrous bundle of thousands of transmembrane alpha-helical coiled coils. Chemoattractants bind at homodimeric interfaces between alpha-helices at the outside surface of the membrane. The protein kinase, CheA, binds to the opposite end that extends into the cytoplasm. CheA catalyzes adenosine 5'-triphosphate (ATP)-dependent phosphorylation of a histidine residue within an associated histidine phosphotransfer or HPt domain. The long-term objective of the proposed research is to use E. coli as a model to determine the molecular logic of signal transduction pathways. How does attractant binding between coiled coil subunits at the outside surface of an E. coli cell control kinase activity in the cytoplasm? How are different phosphorylation-induced conformational states used to transmit information? To address these fundamental questions, the architecture of the coiled coil membrane receptor assemblies will be determined using EM and X-ray crystallographic methods. Dynamical properties and distance constraints will be characterized by fluorescence resonance energy transfer (FRET) measurements. Kinetic studies of CheA phosphotransfer reactions will provide insights concerning kinase regulation and enzymology. In addition to focusing on these structural and kinetic parameters, an effort will be made to characterize the behavioral responses of E. coli in complex environments in order to better assess the information processing capabilities of receptor-kinase signaling complexes. These studies will provide a foundation for understanding general mechanisms that underlie Type I receptor function in both prokaryotic and eukaryotic regulatory systems. [unreadable] [unreadable]